Tubular burner

ABSTRACT

A flame hole member of a tubular burner has a front plate and a rear plate both of sheet metal make. The front plate has a first flame hole in the central portion of the front plate, and a plurality of second flame holes located around a periphery of the first flame hole, each of the second flame holes being of a slit shape in a width below a quenching distance. The rear plate has a first ventilation hole in the central portion of the rear plate, and a plurality of second ventilation holes of smaller diameter than the first ventilation hole, each of the second ventilation holes being located around a periphery of the first ventilation hole. At least one of the rear plate and the front plate is provided with a cylindrical section for introducing into the first flame hole.

BACKGROUND

1. Field of the Invention

The present invention relates to a tubular burner having; a mixing tubeincluding at a rear end thereof an inlet port into which a fuel gas andprimary air flow; a venturi section having a smaller diameter than thediameter of the inlet port; and a tapered tube section having agradually larger diameter from the venturi section toward a front of themixing tube. The tubular burner has a flame hole member with a pluralityof flame holes, the flame hole member being adapted to be fitted into afront end region of the mixing tube.

2. Description of the Related Art

As this kind of burner, there is conventionally known one which isdescribed in U.S. Pat. No. 5,186,620. In the burner described therein, aflame hole member is made of a sintered metal of larger thickness. Aplurality of flame holes which penetrate in the back and forth (i.e.,longitudinal) direction are formed in the flame member so that a mixtureof a fuel gas and primary air (hereinafter also referred to as air-gasmixture) is ejected from these flame holes for combustion.

The flow of the air-gas mixture from the mixing tube toward the flamehole member has a directional component that is directed toward theradially outward direction under the influence of the tapered tubesection. Therefore, if the flame hole member is made smaller inthickness, the flames tend to be spread radially outward. Theabove-mentioned conventional burner, on the other hand, has a flame holemember of larger thickness. As a result, the flow of the air-gas mixtureis rectified at each of the flame holes so as to be directed forward,thereby preventing the flames from getting spread radially outward.

However, in the above-mentioned conventional burner, the flame holemember is made of a sintered metal of higher material cost, therebybringing about a disadvantage of higher cost.

SUMMARY Problems to be Solved by the Invention

In view of the above points, this invention has a problem of providing atubular burner in which a flame hole member is made of a sheet metalplate to thereby reduce the cost, and in which the flames can beprevented from getting spread diametrically outward.

Means for Solving the Problems

In order to solve the above-mentioned problems, this invention providesa tubular burner comprising: a mixing tube inclusive of an inlet port,at a rear end thereof, into which a fuel gas and primary air flow, aventuri section having a smaller diameter than a diameter of the inletport, and a tapered tube section having a gradually larger diameter fromthe venturi section toward a front of the mixing tube; and a flame holemember having a plurality of flame holes and being adapted to be fittedinto a front end region of the mixing tube such that a mixture of thefuel gas and primary air is ejected through the flame holes forcombustion. The flame hole member is made up of a front plate of sheetmetal make, and a rear plate of sheet metal make located behind thefront plate. The front plate has a first flame hole in the centralportion of the front plate, and a plurality of second flame holeslocated around a periphery of the first flame hole, each of the secondflame holes being of a slit shape in a width below a quenching distance.The rear plate has a first ventilation hole in the central portion ofthe rear plate, and a plurality of second ventilation holes of smallerdiameter than the first ventilation hole, each of the second ventilationholes being located around a periphery of the first ventilation hole. Atleast one of the rear plate and the front plate has disposed therein acylindrical section for introducing into the first flame hole themixture of the fuel gas and primary air flowing into the firstventilation hole.

According to this invention, the flame hole member is made of the frontplate and the rear plate, i.e., a total of two plates of sheet metalmake. Therefore, as compared with the above-mentioned conventionalexample in which the flame hole member made of a sintered metal is used,the cost can be reduced. In addition, according to this invention,although the flame hole member is of sheet metal make, the flames can beprevented from getting spread in a radially outward direction.

In other words, according to this invention, by providing thecylindrical section for introducing the air-gas mixture flowing into thefirst ventilation hole toward the first flame hole, the flow of theair-gas mixture directed toward the first flame hole is rectified by thecylindrical section. As a result, the air-gas mixture is ejectedstrongly out of the first flame hole. On the other hand, the flowvelocity of the air-gas mixture that is ejected from the second flameholes of slit shape in the front plate through the second ventilationholes of relatively small diameter in the rear plate can be keptrelatively small. Therefore, the air-gas mixture ejected from the secondflame holes is attracted into the flow of the air-gas mixture that isejected from the first flame hole at a high speed, thereby preventingthe flames from getting spread radially outward.

By the way, should the first flame hole be formed into a pipe-shapedelement that protrudes forward from the front plate without providingthe above-mentioned cylindrical section, there may also be obtained aneffect in that the flow of the air-gas mixture to be ejected from thefirst flame hole is rectified to thereby prevent the flames from gettingspread in the radially outward direction. In this arrangement, however,that heat quantity from the flames which is inputted into thepipe-shaped element of the first flame hole will increase and, as aresult, back firing is likely to occur due to overheating of the firstflame hole. On the other hand, according to this invention, without thenecessity of forming the first flame hole into the pipe-shaped element,there can be obtained an effect of preventing the flames from gettingspread in the radially outward direction. Back firing due to overheatingof the first flame hole can thus be prevented. Further, by forming thesecond flame holes into a slit shape in width that is smaller than thequenching distance, back firing at the second flame holes can also beprevented.

Further, according to this invention, preferably, a rear region of thecylindrical section is gradually reduced in diameter from the firstventilation hole forward, and the cylindrical section in front of therear region is formed into a cylindrical shape of smaller diameter thanthe first ventilation hole. According to this arrangement, in additionto the effect of rectifying the flow of the air-gas mixture, there canalso be obtained an accelerating effect in that the flow velocity of theair-gas mixture is made larger than the flow velocity thereof into thefirst ventilation hole. The spreading of the flames in the radiallyoutward direction can thus be effectively prevented.

Furthermore, preferably, the length of the cylindrical section isequivalent to the longitudinal distance between the rear plate and thefront plate. It is to be noted here that the term “equivalent” includesall of the following cases, i.e.,: the case in which the length of thecylindrical section is the same as the longitudinal distance between therear plate and the front plate; and also the case in which thelongitudinal length between the rear plate and the front plate isslightly shorter than the longitudinal length between the rear plate andthe front plate. In the latter case, a clearance may occur between thefront end of the cylindrical section that is disposed in the rear plateand the front plate, or a clearance may occur between the rear end ofthe cylindrical section that is disposed in the front plate and the rearplate. Even in case such a clearance may occur, the air-fuel gas thatenters the cylindrical section or the air-fuel gas that has flown intothe first ventilation hole will never leak through the clearance intothe space outside the cylindrical section between the front plate andthe rear plate. The case in which such a leak will not occur is alsounderstood to fall under the meaning of “equivalent.” According to thisarrangement, all of the air-gas mixture entering the first ventilationhole is introduced into the first flame hole, and the flow velocity ofthe air-gas mixture to be ejected from the first flame hole becomeslarger. As a result, the flames can be effectively prevented fromgetting spread in the radially outward direction.

Further, in case the cylindrical section is disposed in the rear plate,the diameter of the first flame hole is preferably larger than an innerdiameter at the front end of the cylindrical section. According to thisarrangement, the air-gas mixture entering the cylindrical section cansmoothly be ejected from the first flame hole without being hindered bythe front plate. As a result, the flow velocity of the air-gas mixturethat is ejected from the first flame hole becomes larger and also thepressure loss becomes smaller.

By the way, in case the second flame holes are of slit shape elongatedin a radial direction of the front plate, preferably a plurality ofinner second flame holes are formed in a portion, closer to the firstflame hole, of the front plate at a circumferentially equal pitch, and aplurality of outer second flame holes are formed while beingcircumferentially deviated by half a pitch from the inner second flameholes so that the outer second flame holes are located between: such anintermediate portion in the front plate as is located in a diametricallyinner end and a diametrically outer end of the inner second flame holes;and such a portion in the front plate as is located diametricallyoutward of the diametrically outer end of the inner second flame holes.According to this arrangement, the second flame holes can be disposed inthe front plate in a well-balanced manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of a tubular burner according to a firstembodiment of this invention.

FIG. 2 is a perspective view of the tubular burner according to thefirst embodiment of this invention.

FIG. 3 is a partly cut-away perspective view of the tubular burneraccording to the first embodiment of this invention.

FIG. 4 is a partly cut-away perspective view of a tubular burneraccording to a second embodiment of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 reference numeral 1 denotes a tubular burner according to anembodiment of this invention. This burner 1 is used as a heat source ofa heating appliance, and is disposed so as to lie opposite to an inletend of a heat exchange pipe P which performs heat exchanging with roomair.

The burner 1 is made up of a mixing tube 2, and a flame hole member 3which is adapted to be fitted into a front end region of the mixing tube2. Also with reference to FIGS. 2 and 3, the mixing tube 2 has: an inletport 21 at a rear end thereof; a venturi section 22 which is reduced indiameter relative to the inlet port 21; and a tapered tube section 23which is gradually increased in diameter from the venturi section 22forward. In this arrangement, a fuel gas ejected from a gas nozzle (notillustrated) which is disposed so as to face the inlet port 21, andprimary air flow from the inlet port 21 into the mixing tube 2 so that amixture of fuel gas and primary air is generated within the mixing tube2. The mixing tube 2 is made of a thin sheet metal plate, and is formedby combining together two sheet metal plates 2 a, 2 a made, e.g., ofpress-formed stainless steel, and the like.

Although not illustrated, a plurality of the tubular burners 1 aredisposed in parallel with one another. At the front end region of thetwo sheet metal plates 2 a, 2 a that constitute the mixing tube 2, thereis each formed a dented portion 2 b in a manner to be away from theother sheet metal plate 2 a. The clearance to be generated between thetwo sheet metal plates 2 a, 2 a by means of these dented portions 2 bconstitutes a slit-shaped carry-over flame hole 2 c which causes flamesto be carried over to the adjoining burners.

The front end region of the mixing tube 2 is formed in a cylindricalshape which is elongated forward from an enlarged-diameter region 23 aof a curved shape at the front end of the tapered tube section 23. Theflame hole member 3 to be fitted into the front end region of the mixingtube 2 is constituted by a front plate 4 which is formed of a sheetmetal plate of stainless steel make and the like, and a disk shaped rearplate 5 which is formed of a sheet metal of stainless steel make and thelike and which is located rearward of the front plate 4.

The front plate 4 has a tubular member 4 a which is elongated backwardfrom a circular disk-shaped front portion so as to be fitted into theinner circumference of the front portion of the mixing tube 2. The frontplate 4 is provided with a first flame hole 41 in the central portion atthe front face of the front plate 4, and a plurality of second flameholes 42 which are located around the periphery of the first flame hole41 and which are in slit shape in a width (e.g., 0.7 mm) below aquenching distance.

The second flame holes 42 are formed into slits which are elongated inthe radial direction of the front plate 4. In case this kind of secondflame holes 42 are formed simply at an equal pitch in thecircumferential direction, the distance between the adjoining secondflame holes 42, 42 becomes too large toward the radially outwardportion. As a solution, in this embodiment, a plurality of inner secondflame holes 42in are formed in a portion, closer to the first flamehole, of the front plate 4 at a circumferentially equal pitch. Also, aplurality of outer second flame holes 42out are formed circumferentiallydeviated by half a pitch from the inner flame holes 42in so as to belocated between: such an intermediate portion, in the front plate 4, asis located in a diametrically inner end and a diametrically outer end ofthe inner second flame holes 42in; and such a portion, in the frontplate 4, as is located diametrically outward of the diametrically outerend of the inner second flame holes 42in. According to this arrangement,the second flame holes 42 can be disposed in the front plate 4 in awell-balanced manner.

The second flame holes 42 may alternatively be formed in a slit shapeelongated in the circumferential direction of the front plate 4. Inother words, on a plurality of circles that are coaxial with the firstflame hole 41, a plurality of the second flame holes may be formed.

Further, at a front end of the tubular member 4 a of the front plate 4,there is formed a rounded corner portion 4 b which is away from theinner peripheral surface of the front end region of the mixing tube 2.At the rounded corner portion 4 b there is formed a plurality of flameretention holes 43 of a slit shape at a circumferential distance fromone another.

The rear plate 5 is provided with a first ventilation hole 51 in thecentral portion of the rear plate 5, and a plurality of secondventilation holes 52 which are of a smaller diameter than that of thefirst ventilation hole 51 and which are located around the periphery ofthe first ventilation hole 51. The rear plate 5 has further formedtherein a cylindrical section 53 which projects forward from the holeedge of the first ventilation hole 51. It is thus so arranged that theair-gas mixture flowing into the first ventilation hole 51 is introducedinto the first flame hole 41 through the cylindrical section 53.

The rear region of the cylindrical section 53 is gradually reduced indiameter from the first ventilation hole 51 toward the front side. Thatportion of the cylindrical section 53 which lies in front of the rearregion (i.e., the front portion of the cylindrical section 53) is formedinto a cylindrical shape of smaller diameter than that of the firstventilation hole 51. Further, the length of the cylindrical section 53is the same as the longitudinal distance between the rear plate 5 andthe front plate 4 so that no clearance in the longitudinal directionoccurs between the front end of the cylindrical section 53 and the frontplate 4. Furthermore, the diameter of the first flame hole 41 is largerthan the inner diameter of the front end of the cylindrical section 53.In this embodiment, the diameter of the first flame hole 41 is madesubstantially equal to the outer diameter of the front end of thecylindrical section 53.

According to the above-mentioned tubular burner 1 of this embodiment,the flame hole member 3 is constituted by the front and the rear, atotal of two, plates 4, 5 of sheet metal make. Therefore, as comparedwith the conventional example in which a flame hole member made of asintered metal is used, the cost can be reduced. Further, if the flamehole member 3 is made of a sheet metal plate, the air-gas mixture of thefuel gas and the primary air is ejected with a directional componentthat is directed in a radially outward direction under the influence ofthe tapered tube section 23 of the mixing tube 2, and the flames arelikely to be spread in the radially outward direction. However, in thisembodiment, the flames can be prevented from spreading radially outward,whereby the flames can surely be introduced into the heat exchange pipeP. A description will now be made of the reasons.

In this embodiment, because the cylindrical section 53 is disposed tointroduce the air-gas mixture flowing into the first ventilation hole 51toward the first flame hole 41, the flow of the air-gas mixture directedto the first flame hole 41 is rectified by the cylindrical section 53.As a result, the air-gas mixture is forcibly ejected forward from thefirst flame hole 41. On the other hand, the flow velocity of the air-gasmixture to be ejected from the slit-shaped second flame holes 42 in thefront plate 4 via the second ventilation holes 52 of relatively smallerdiameter in the rear plate 5 is kept relatively low. As a result, due toBernoulli law, the air-gas mixture ejected from the second flame holes42 is attracted by the flow of the air-gas mixture ejected at a highspeed from the first flame hole 41. Consequently, the flames to beformed by the combustion of the air-fuel mixture ejected from the secondflame holes 42 are combined into the flame to be formed by thecombustion of the air-fuel mixture ejected from the first flame hole 41,whereby aggregated flames Fa elongated forward are formed and the flamescan be prevented from getting spread radially outward.

In this embodiment, since the front portion of the cylindrical section53 is formed into a cylindrical shape having a diameter smaller thanthat of the first ventilation hole 51, there can be obtained anaccelerating effect in which the velocity of the air-gas mixture islarger than the incoming velocity into the first ventilation hole 51, inaddition to the rectifying effect of the air-gas mixture. Therefore, thevelocity of the air-gas mixture ejected from the first flame hole 41becomes larger, whereby the radially outward spreading of the flames caneffectively be prevented.

In case the cylindrical section 53 is not provided but the first flamehole is formed into a pipe-shaped element that protrudes forward fromthe front plate, there can also be obtained an effect in that the flowof air-gas mixture ejected from the first flame hole is rectified andthat the spreading of the flames in the radially outward direction canbe prevented. In this arrangement, however, the amount of heat from theflames to be inputted into the pipe-shaped element in the first flamehole becomes too large, and backfiring is likely to occur due tooverheating of the first flame hole. In this embodiment, on the otherhand, there can be obtained an effect in that, without forming the firstflame hole 41 into a pipe-shaped element, the radially outward spreadingof the flames can be prevented. Backfiring due to overheating of thefirst flame hole 41 can thus be prevented. In addition, by forming thesecond flame holes 42 into slit shape of a width below the quenchingdistance, backfiring in the second flame hole 42 can also be prevented.

In place of the slit-shaped second flame holes 42, backfiring cansimilarly be prevented by forming, as the second flame holes, amultiplicity of circles of diameter smaller than the quenching distance.It is, however, advantageous to arrange the second flame holes 42 intoslits like in this embodiment, because the fabrication becomes easierand the pressure loss can be reduced.

It is also possible to make the length of the cylindrical section 53smaller than the longitudinal distance between the rear plate 5 and thefront plate 4. In this arrangement, however, there will occur aclearance in the longitudinal direction between the front end of thecylindrical section 53 and the front plate 4. Part of the air-gasmixture flowing into the cylindrical section 53 may leak through thisclearance into the space outside the cylindrical section 53 between thefront plate 4 and the rear plate 5. As a result, the flow velocity ofthe air-gas mixture ejecting from the first flame hole 41 will belowered. In the arrangement of this invention, on the other hand, thelength of the cylindrical section 53 is made equal to the longitudinaldistance between the rear plate 5 and the front plate 4 so that noclearance in the longitudinal direction occurs between the front end ofthe cylindrical section 53 and the front plate 4. As a result, all ofthe air-gas mixture flowing into the first ventilation hole 51 will beintroduced into the first fame hole 41. In conjunction with theabove-mentioned accelerating function, the velocity of the air-fuelmixture ejected from the first flame hole 41 becomes still faster, andthe radially outward spreading of the flames can more effectively beprevented.

In this embodiment, an arrangement has been made that the length of thecylindrical section 53 is set to be the same as the longitudinaldistance between the rear plate 5 and the front plate 4. However, itneed not be limited to such an arrangement. In other words, take forexample a case in which the length of the cylindrical section 52 isslightly smaller than the longitudinal distance between the rear plate 5and the front plate 4 and, as a result, a clearance occurs in thelongitudinal direction between the front end of the cylindrical section53 and the front plate 4. Even in such a case, if the air-gas mixtureflowing into the cylindrical section 53 does not leak into the spaceoutside the cylindrical section 53 between the front plate 4 and therear plate 5, there can be obtained a similar effect as the onedescribed above.

Further, in this embodiment, the diameter of the first flame hole 41 ismade larger than the inner diameter of the front end of the cylindricalsection 53. Therefore, the air-gas mixture flowing into the cylindricalsection 53 is smoothly ejected from the first flame hole 41 withoutbeing disturbed by the front plate 4. The velocity of the air-gasmixture ejected from the first flame hole 41 becomes larger and thepressure loss becomes smaller.

Further, the air-gas mixture ejected from each of the flame retentionholes 43 gets collided with the inner circumference at the front end ofthe mixing tube 2, and is then diffused in the circumferential directionin the annular clearance that is generated between the rounded cornerportion 4 b and the inner circumference at the front end of the mixingtube 2. The air-gas mixture is thereafter ejected forward from thisclearance. Since the velocity of ejection of the air-gas mixture fromthis clearance is lowered due to the collision of the air-gas mixturewith, and diffusion thereof into, the inner circumference of the frontend of the mixing tube 2, there can be formed flames Fb that are hard tobe lifted off, thereby securing the flame retention property.

Now, a description will be made of a second embodiment of this inventionas shown in FIG. 4. The basic construction of the second embodiment isnot particularly different from that of the first embodiment. The samereference numerals as those in the first embodiment are thereforeassigned to the similar members and the parts. The difference of thesecond embodiment from the first embodiment is that the diameter of thefirst flame hole 41 is made larger than the outer diameter of the frontend of the cylindrical section 53.

Also in the second embodiment, the air-gas mixture entering thecylindrical section 53 is smoothly ejected from the first flame hole 41without being disturbed by the front plate 4. Therefore, in the samemanner as in the first embodiment, the flow velocity of the air-gasmixture ejected from the first flame hole 41 is accelerated and also thepressure loss becomes smaller.

Descriptions have so far been made of embodiments of this invention withreference to the accompanying drawings. This invention is however notlimited to the above embodiments. Although the cylindrical section 53 isformed in the rear plate 5 in the above embodiments, the followingarrangement may also be made. For example, the front plate 4 is providedwith a cylindrical section that protrudes backward from the hole edge ofthe first flame hole 41 toward the first flame hole 51 so that theair-gas mixture flowing into the first flame hole 51 is introduced intothe first flame hole 41 through the cylindrical section. In thisarrangement, preferably, the length of the cylindrical section is madeequal to the longitudinal distance between the rear plate 5 and thefront plate 4 so that, between the rear end of the cylindrical sectionand the rear plate 5, there occurs no clearance which allows air-gasmixture flowing into the first ventilation hole 51 to leak into thespace outside the cylindrical section between the rear plate 5 and thefront plate 4. It is also possible to make the length of the cylindricalsection larger than the longitudinal distance between the rear plate 5and the front plate 4 so that the rear end region of the cylindricalsection is fitted into the first ventilation hole 51. Further, in casethe cylindrical section is disposed in the front plate 4, it ispreferable to gradually reduce the diameter of the rear region of thecylindrical section from the first ventilation hole 51 toward the frontside so that the front portion of the cylindrical section is formed intoa cylindrical shape having a smaller diameter than the first ventilationhole 51 but the same diameter as the first flame hole 41. However, incase the cylindrical section is formed in the front plate 4, thefabrication of the cylindrical section becomes troublesome. Therefore,it is advantageous to form the cylindrical section 53 in the rear plate5 as in the above-mentioned embodiments.

In addition, the following arrangement may also be employed. Forexample, the rear plate 5 is provided with a cylindrical section thatprotrudes forward from the edge of the first ventilation hole 51. Thefront plate 4 is provided with a cylindrical section that protrudesbackward from the edge of the first flame hole 41. Then, both thecylindrical sections are fitted between the rear plate 5 and the frontplate 4 so that the air-gas mixture flowing into the first ventilationhole 51 is introduced into the first flame hole 41 through both thecylindrical sections. It is possible to provide at least one of thefront plate 4 and the rear plate 5 with an independent cylindricalsection other than these plates 4, 5. Although the mixing tube 2 ofsheet metal make is used in the above-mentioned embodiments, it is alsopossible to use a mixing tube made of a cast steel. Further, the flameretention holes 43 in the above-mentioned embodiments may be omitted. Inaddition, in the above-mentioned embodiments, this invention was appliedto a tubular burner for heating appliances. This invention can, however,be applied to tubular burners which are used in a combustion equipmentother than a heating appliance.

What is claimed is:
 1. A tubular burner comprising: a mixing tubeinclusive of an inlet port, at a rear end thereof, into which a fuel gasand primary air flow, a venturi section having a smaller diameter than adiameter of the inlet port, and a tapered tube section having agradually larger diameter from the venturi section toward a front of themixing tube; and a flame hole member having a plurality of flame holesand being adapted to be fitted into a front end region of the mixingtube such that a mixture of the fuel gas and primary air is ejectedthrough the flame holes for combustion, the flame hole member being madeup of a front plate of sheet metal make, and a rear plate of sheet metalmake located behind the front plate, the front plate having a firstflame hole in a central portion of the front plate, and a plurality ofsecond flame holes located around a periphery of the first flame hole,each of the second flame holes being of a slit shape in a width below aquenching distance, the rear plate having a first ventilation hole in acentral portion of the rear plate, and a plurality of second ventilationholes of smaller diameter than the first ventilation hole, each of thesecond ventilation holes being located around a periphery of the firstventilation hole, at least one of the rear plate and the front platehaving disposed therein a cylindrical section for introducing into thefirst flame hole the mixture of the fuel gas and primary air flowinginto the first ventilation hole.
 2. The tubular burner according toclaim 1, wherein a rear region of the cylindrical section is graduallyreduced in diameter from the first ventilation hole forward, and whereinthe cylindrical section in front of the rear region is formed into acylindrical shape of smaller diameter than the first ventilation hole.3. The tubular burner according to claim 1, wherein a length of thecylindrical section is equivalent to a longitudinal distance between therear plate and the front plate.
 4. The tubular burner according to claim1, wherein the cylindrical section is disposed in the rear plate, andwherein a diameter of the first flame hole is larger than an innerdiameter at the front end of the cylindrical section.
 5. The tubularburner according to claim 1, wherein the second flame holes are of slitshape elongated in a radial direction of the front plate, wherein aplurality of inner second flame holes are formed in a portion, closer tothe first flame hole, of the front plate at a circumferentially equalpitch, and wherein a plurality of outer second flame holes are formedwhile being circumferentially deviated by half a pitch from the innersecond flame holes so that the outer second flame holes are locatedbetween: such an intermediate portion in the front plate as is locatedin a diametrically inner end and a diametrically outer end of the innersecond flame holes; and such a portion in the front plate as is locateddiametrically outward of the diametrically outer end of the inner secondflame holes.
 6. The tubular burner according to claim 2, wherein thesecond flame holes are of slit shape elongated in a radial direction ofthe front plate, wherein a plurality of inner second flame holes areformed in a portion, closer to the first flame hole, of the front plateat a circumferentially equal pitch, and wherein a plurality of outersecond flame holes are formed while being circumferentially deviated byhalf a pitch from the inner second flame holes so that the outer secondflame holes are located between: such an intermediate portion in thefront plate as is located in a diametrically inner end and adiametrically outer end of the inner second flame holes; and such aportion in the front plate as is located diametrically outward of thediametrically outer end of the inner second flame holes.
 7. The tubularburner according to claim 3, wherein the second flame holes are of slitshape elongated in a radial direction of the front plate, wherein aplurality of inner second flame holes are formed in a portion, closer tothe first flame hole, of the front plate at a circumferentially equalpitch, and wherein a plurality of outer second flame holes are formedwhile being circumferentially deviated by half a pitch from the innersecond flame holes so that the outer second flame holes are locatedbetween: such an intermediate portion in the front plate as is locatedin a diametrically inner end and a diametrically outer end of the innersecond flame holes; and such a portion in the front plate as is locateddiametrically outward of the diametrically outer end of the inner secondflame holes.
 8. The tubular burner according to claim 4, wherein thesecond flame holes are of slit shape elongated in a radial direction ofthe front plate, wherein a plurality of inner second flame holes areformed in a portion, closer to the first flame hole, of the front plateat a circumferentially equal pitch, and wherein a plurality of outersecond flame holes are formed while being circumferentially deviated byhalf a pitch from the inner second flame holes so that the outer secondflame holes are located between: such an intermediate portion in thefront plate as is located in a diametrically inner end and adiametrically outer end of the inner second flame holes; and such aportion in the front plate as is located diametrically outward of thediametrically outer end of the inner second flame holes.